(D) says, "Large bits of cosmic rock periodically enter Earth’s atmosphere, raising large amounts of dust from Earth’s surface."
With (D), we'd know that large bits of rock raise lots of dust into the atmosphere when they hit Earth. However, is this related to temperatures on Earth? We have no clue whether or not it is. Because (D) doesn't provide support for the climatologists' hypothesis, it is the correct answer choice, as we're looking for the one answer that does not lend support for their hypothesis.
Does this make sense? Let us know if you have any other questions!